This invention relates to color and monochrome electronic imaging, and more particularly to the generation of color and monochrome prints of images represented by electronic signals.
Color cathode ray tubes (CRTs) are by far the most ubiquitous display technology. They are used in applications ranging from home video displays to high resolution computer aided design. Presently, most analog video images have medium spatial resolution (typically 512 horizontal by 480 vertical pixels or less), however higher resolution displays are becoming more available, particularly in commercial environments. CRTs are also characterized by nearly continuous intensity, hue and saturation.
In order to take full advantage of the video display technology, a compatible printing technology is required to transport video images onto a hard-copy medium, such as color prints. Applications that could benefit from color pictures include everyday business memoranda, catalog sales, real estate sales, point of sale terminals, Videotex applications, law enforcement, electronic photography and home entertainment.
The currently available color hard-copy printers employ diverse technologies. Ink jet printing uses fast drying ink droplets (yellow, magenta, cyan and possibly black) are thrust onto paper, where they combine to form a color image. The major disadvantage of ink jet printing is the complex hardware, which is also expensive and prone to failure (clogging). Thermal transfer also uses two techniques: direct transfer and indirect transfer. In monochrome direct transfer thermal printing, a print head raises the temperature of a chemically impregnated paper to a "blush point", whereupon two chemicals in the paper combine to form the image. In indirect transfer, a print head liquifies or sublimes a wax-ink on a polymer substrate and transfers the ink onto paper. Simple thermal transfer printers only print 8 colors. Thermal sublimation printers have continuous tone capability. The major disadvantage of thermal transfer printing is the speed and registration difficulties with the three colors. Electrostatic color printing is like the familiar black-and-white printing that was originally introduced by Xerox. It differs only in that each of the three colors must be handled independently, with separate exposures, toners, fixers, etc. The main disadvantage of electrostatic color printing is the complexity, cost and registration requirements associated with the triple optical systems and the coupled hardware for "developing" the prints. High quality color printing is achived via color photography by using a CRT and color separation filters. Conventional film processing techniques are very slow, while instant photography is convenient but suffers from high print cost, short film shelf life, and messy disposable components.
The two recent advances in light-sensitive recording materials may make it possibe to construct video printers which come close to obtaining the image quality of standard photographic printers while reducing print cost and processing complexity. The Mead Corporation (Dayton, Ohio) has developed a photosensitive "paper" which is based on microencapsulation and photopolymerization technology. The panchromatic version of the paper is capable of producing color continuous tone images when exposed to light in the visible light between 400 and 700 nm. This micronencapsulation technology is described in U.S. Pat. 4,399,209, issued to Sanders et al. on Aug. 16, 1983, and in "Photocapsule Process for Hardcopy Output," Electronic Imaging, pp. 35-41, Oct. 1984. The photo-sensitive "paper", which has about 1/1000 the sensitivity of standard photographic film, is processed by placing the exposed microcapsule sheet against a receiver sheet and rupturing the microcapsules with pressure rollers. The pressure releases a dye precursor which forms an image of a receiving color print sheet by means of chemical reactions with an acid on the receiver sheet. Dye formation occurs by means of a chemical reaction with an acid on the receiver sheet within several seconds if the color print sheet is heated to about 100.degree. C. This is a dry process with the only disposable material being the microcapsule sheet. The system uses only organic dyes and no silver, and thus the cost per print is expected to be low.
The 3M Company (St. Paul, Minn.) has developed a color version of their dry silver paper which has been used for many years in monochrome CRT video printing systems. The color dry silver paper is exposed at the visible wavelengths and has a sensitivity of about 1/100th that of standard instant color films. It is processed by heating the exposed paper to about 132.degree. C. for about 10 seconds, and involves no disposables.
The key to success for these newly developed light sensitive recording materials, as well as for the more established recording materials, is a compact and fast printer capable of producing the high light levels required with spatial resolution and a color gamut that is compatible with color CRT displays. The printer should have a low initial cost, a low operating cost, and be inherently reliable. None of the known approaches satisfy all of these needs.